Your observations are correct -- currently it is not possible to change the gauge background via simple property setter. You need to customize the LinearGauge template in order to achieve the desired effect as the background is implemented via Path
(that enables drawing of curves and complex shapes).
Alternatively, you can use one of the other two predefined themes (Office_Black, Vista). You can find attached a sample application that demonstrates both approaches to get you started. Also, you can find more information on gauge layout composition here
Hope this helps.
the Telerik team
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